Conventionally, there is known a mobile communication system which can include a 2G/3G radio access system (UTRAN/GERAN (UTRAN: Universal Terrestrial Radio Access Network, GERAN: GSM EDGE Radio Access Network)) and an LTE (Long Term Evolution) radio access system (E-UTRAN: Evolved Universal Terrestrial Radio Access Network).
In such a mobile communication system, a UE (User Equipment, a mobile station) capable of accessing both the UTRAN/GERAN and the E-UTRAN can be provided with voice communication by either the UTRAN-GERAN or the E-UTRAN.
Such a mobile communication system is configured such that an IMS (IP Multimedia Subsystem, a service control network) can select a domain which is to provide voice communication to a UE, through “T-ADS (Terminated-Access Domain Selection)” (see Non-patent Documents 1 and 2).
With reference to FIG. 4, a description is given of an operation of selecting a domain which is to provide voice communication to a UE, through the “T-ADS.”
As shown in FIG. 4, in Step S501, an S-CSCF (Serving Call Session Control Function, a call session control server apparatus) in the IMS receives a terminating signal for voice communication (Invite @ Voice) directed to the UE. In Step S502, the S-CSCF sends the terminating signal for voice communication (Invite) to an SCC-AS (Service Centralization and Continuity Application Server, an application server apparatus) in the IMS.
In Step S503, the SCC-AS sends an inquiry signal to an HSS (Home Subscriber Server, a subscriber management server apparatus).
In Step S504, in response to the inquiry signal, the HSS sends a domain inquiry signal for inquiring about a domain in which the UE is located, to an SGSN (Serving GPRS Support Node, a packet switch) and an MME.
In Step S505, the HSS notifies the SCC-AS of the domain in which the UE is located, by sending the SCC-AS a response signal in response to the inquiry signal described above.
In Step S506, based on the response signal, the SCC-AS determines a domain to which the terminating signal for voice communication directed to the UE should be transferred, and sends the S-CSCF a terminating domain designation signal instructing the S-CSCF to transfer the terminating single for voice communication directed to the UE to the domain thus determined.
In Step S507, via the domain (radio access network) designated by the terminating domain designation signal, the S-CSCF sends the UE a terminating call indication signal indicating reception of the terminating signal for voice communication directed to the UE.